1. Field of the Invention
The present invention relates to a process and an apparatus for suspension polymerization. More particularly, it relates to a process and an apparatus for suspension polymerization which can produce polymer particles having a uniform particle size.
2. Description of Prior Art
As a process for suspension polymerization by which polymer particles having a uniform particle size can be produced, known is a suspension polymerization process which comprises steps of forming a fluidized bed comprising an aqueous dispersion medium and monomer liquid droplets in a tower type reactor and suspension polymerizing the monomer. That is, the monomer liquid droplets having a uniform size are formed in the aqueous dispersion medium containing a suspension stabilizer for the monomer liquid droplets and then a fluidized bed of the medium containing the monomer liquid droplets is formed by utilizing the difference of specific gravity between the monomer liquid and the dispersion medium (usually the monomer liquid is lighter than the aqueous dispersion) in a tower type reactor in which a downflow of the aqueous dispersion medium is formed. Thereafter, the monomer is polymerized without applying strong shear to the monomer liquid droplets, whereby the monomer can be polymerized with avoiding merging or splitting of the monomer liquid droplets.
For example, Japanese Patent Publication No. 36510/1978 or U.S. Pat. No. 3,922,255 discloses an suspension polymerization process in which a monomer liquid is passed thorough an orifice or orifices to form droplets having a uniform size, charged in a reactor and partially polymerized in the reactor to such extent that the droplets are not merged or split with gentle shear. Then, the partially polymerized monomer liquid droplets are removed from a bottom of the reactor together with the aqueous dispersion medium and introduced in a solid-liquid separator which is connected with the reactor, in which the partially polymerized droplets are flotation separated and concentrated. The separated droplets are charged in an agitated reactor and suspension polymerized with avoiding merging or splitting to give a polymer with a high polymerization conversion.
Japanese Patent Publication No. 91701/1983 or U.S. Pat. No. 4,487,898 discloses a suspension polymerization process in which a laminar flow of a monomer liquid is gushed from a vibrating orifice to form monomer liquid droplets having a uniform size, introduced in a tower type reactor and partially polymerized to such extent that the specific gravity of the monomer liquid droplets becomes close to that of the aqueous dispersion medium. A slurry comprising the partially polymerized monomer liquid droplets and the dispersion medium is circulated through the tower type reactor and a precipitator, during which the partially polymerized monomer liquid droplets having a specific gravity larger than that of the dispersion medium is precipitated and concentrated in the precipitator, and then charged in a second tower type reactor to further continue suspension polymerization, whereby a polymer is continuously produced with a high polymerization conversion.
Such conventional processes require the floatation separator or the precipitator which is connected with the reactor and should be operated very carefully, since the dispersion medium has substantially the same specific gravity as water the gravity of which is about one (1) around room temperature and the partially polymerized soft droplets formed in the reactor is to be separated from the aqueous dispersion medium. In addition, to further polymerize the partially polymerized droplets to such extent that the droplets can be dehydrated, dried and stored, a specially designed reactor which requires special attention for mixing and stirring should be used to further polymerize the partially polymerized soft droplets.
In the conventional processes, the partially polymerized droplets with a conversion of from about 30 to 60% are so soft that they tend to merge with each other or stick to a wall of the apparatus where a flow rate is small and the stream is retained in the floatation separator or the precipitator. Therefore, the apparatus should be very carefully operated for stable operation. In addition, since the partially polymerized droplets are further polymerized, the agitated type reactor should have specially designed agitating blades and be operated with a limited range of rotational speed in order to prevent splitting of the partially polymerized soft droplets and formation of smaller droplets. When the tower type reactor is used, upward flow of the aqueous dispersion medium should be formed therein, and a flow rate of said upward flow and a slope angle of a conical bottom portion of the reactor should be carefully arranged. Further, mixing of the monomer liquid droplets having low polymerization conversion will cause merging due to retention in an upper portion of the reactor.
The conventional processes in which the partially polymerized monomer liquid droplets are separated from the aqueous dispersion medium and concentrated, and then further polymerized to achieve higher conversion require not only complicated apparatus because of the properties of the partially polymerized droplets, but also involve technical problems such as careful operation of the apparatus.